Donald Trump said on Twitter, "Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!" (Credit: AFP / Getty Images / Josh Edelson)

Donald Trump said Sunday that President Barack Obama should resign from office because he didn't say the words "radical Islam" in response to the Orlando nightclub massacre that claimed the lives of 49 people.

"Because our leaders are weak, I said this was going to happen — and it is only going to get worse," Trump said in a statement. "I am trying to save lives and prevent the next terrorist attack. We can't afford to be politically correct anymore."

Earlier, Trump tweeted that he was "right on radical Islamic terrorism."

"Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!"

Later, he tweeted, "What has happened in Orlando is just the beginning. Our leadership is weak and ineffective. I called it and asked for the ban. Must be tough."

Trump was presumably referring to a ban on Muslim entry into the U.S., which he has repeatedly touted over the course of his presidential campaign.

The gunman in Sunday's massacre was identified as Omar Mateen, 29, a U.S. citizen who was born in New York and lived in Florida. Police said Mateen killed 49 people and wounded 53 others at Pulse nightclub in what has become the deadliest mass shooting in U.S. history.

The FBI said that Mateen called 911 on Sunday and expressed his support for Islamic State, but cautioned that a suspected link to radical Islamism required further investigation.